1. Field of the Invention (Technical Field)
The present invention relates to apparatus for detecting substances, particularly controlled or dangerous substances, and specifically for detecting small quantities of substances upon a person or object.
2. Background Art
The continued use of explosives by terrorists has prompted the United States Federal Aviation Administration and similar organizations worldwide to pursue the development of various explosives detection systems for the screening of airline passengers and baggage. The use of traditional x-ray based systems for bulk detection has fallen into some disfavor due to significant privacy and public health concerns. In the case of screening persons, moreover, trace detection systems, rather than bulk detection, are of increasing interest for the identification of individuals who recently have handled explosives materials, to alleviate the added risk such persons may pose to mass transit passengers. Any apparatus or method for detecting trace amounts of a particular substance on persons must be relatively non-invasive and physically innocuous--preferably involving no physical contact--so as not unduly to infringe upon the physical privacy of the person being screened.
Any practicable detection apparatus must also be capable of rapid operation. Particulate collection by wiping or brushing a surface to be tested improves reliability of the test, but consumes too much time on a per-test basis to be practical in busy international airports and the like. An effective detection apparatus accordingly must be capable of completing an entire test cycle in a matter of a few seconds, but also must be able reliably to perform numerous test cycles repeatedly for prolonged periods of time. Thus, few moving parts and simplicity of structure and operation are demanded for long-term durability after thousands of test cycles.
Thus, a viable explosives detection portal must overcome at least two primary impediments. First, a practical detection device must accommodate the physical properties of explosives chemicals that make explosives vapor detection difficult, namely, the extremely low vapor pressure of explosives commonly used by terrorists. For example, pentaerythritol tetranitrate (PETN), cyclonite (RDX), C-4, and Semtex, potent and commonly used explosives, have vapor pressures of only a few parts per trillion under standard pressure and temperature. Low vapor pressures compel the use of highly sensitive detectors in the apparatus.
Additionally, a practical detector must have a very short screening time. It is widely believed that a commercially acceptable detection portal must screen people at a rate of no more than about ten seconds per person in order to screen all persons using the typical airport boarding concourse without unacceptable delay and frustration. Due to the time constraint, the problem of low explosives vapor pressure cannot be overcome merely by increasing the length of time the explosives vapors/particles are collected in the portal detector.
Currently, explosives detection at large airports and other facilities is performed mainly by the use of bomb-sniffing dogs. The use of dogs nevertheless has proven to be too time consuming and somewhat unpredictable as a long-term solution for safeguarding the traveling public.
The traveling public is accustomed to the use of portal-type metal detectors, which detectors require no physical contact and little time to operate and thus serve as a plausible paradigm for explosives detection as well. A need remains, therefore, for a "walk-through, no-contact" detection portal apparatus through which persons and/or objects may quickly be passed, yet which reliably detects the presence on the person or object of dangerous chemicals such as explosives or illegal drugs.
Against the foregoing background, the present invention was developed. The detection portal described below overcomes the significant impediments posed to a walk-through explosives detection portal.